1. Field of the Invention
The present invention relates to package structures, and more particularly, to a package structure with ESD (electrostatic discharge) and EMI (electromagnetic interference) preventing functions.
2. Description of Related Art
Along with the rapid development of science and technology, electronic products are becoming lighter, thinner, shorter and smaller so as to make it easy for consumers to carry and use the products. A method for forming a semiconductor package generally comprises: mounting and electrically connecting a semiconductor chip to a carrier such as a substrate; encapsulating the semiconductor chip and the carrier with an encapsulant such as an epoxy resin so as to protect the semiconductor chip and the carrier against external moisture or pollutants; and covering the encapsulant with a lid member such as a metal shell or, alternatively, covering the semiconductor chip and the carrier with a lid member such as a metal shell, thereby protecting the semiconductor chip from being damaged by external influences, such as ESD, and blocking internal and external EMI and EMC (electromagnetic compatibility).
In such a package, the lid member is electrically connected to a ground structure of the package and further electrically connected to a system ground to thereby conduct away external electromagnetic radiation and electrostatic charges.
FIGS. 1A and 1B show a semiconductor package with a meshed metal shield as disclosed by U.S. Pat. No. 5,166,772. Referring to FIGS. 1A and 1B, a meshed metal shield 12 is disposed on a substrate 10 to cover a chip 11, and an encapsulant 13 encapsulates the metal shield 12 and the chip 11. The metal shield 12 is electrically connected to a ground circuit 14 of the substrate 10 so as to shield EMI generated by the chip 11 or an external device.
FIG. 2 shows another semiconductor package as disclosed by U.S. Pat. No. 6,187,613. Referring to FIG. 2, a chip 11 is mounted to a substrate 10 through a plurality of conductive bumps 15 in a flip-chip manner, a metal foil 16 is attached to the substrate 10 and the chip 11, and an encapsulant 13 is filled between the metal foil 16 and the substrate 10. Therein, the metal foil 16 shields EMI generated by the chip 11 or an external device.
However, since the meshed metal shield or the metal foil is electrically connected to the ground circuit of the chip and active/passive components, when the semiconductor package is disposed on a circuit board, if the meshed metal shield or the metal foil carries electrostatic charges, the electrostatic charges can be conducted along the ground circuit to the circuit board, the chip and the active/passive components. When the electrostatic charges are conducted to the chip and the active/passive components, an electrostatic discharge can occur, thereby easily causing damage of the chip and the active/passive components.
Further, due to too long electrical connecting path from the metal shield or the metal foil to the system ground, especially for a substrate 10 with less than six layers, the grounding effect is adversely affected such that electrostatic charges cannot be released rapidly, thereby easily causing damage of the chip or other active/passive components.
Therefore, it is imperative to provide a package structure so as to protect an internal chip or an active/passive component from being damaged by static electricity and having preferred ESD and EMI preventing functions.